1. Field of the Invention
The present invention relates to a magneto-optical recording apparatus operated in the magnetic modulation method.
2. Related Background Art
There are heretofore well known magneto-optical recording apparatus which can record information signals in a magneto-optical disk by the magnetic modulation method. FIG. 1 shows the schematic construction of a conventional magneto-optical recording apparatus of such type. In FIG. 1, reference numeral 1 designates a disk, 2 an optical head, and 3 a magnetic head. The magnetic head 3 is a floating magnetic head provided with a slider, which is held above the upper surface of disk 1 by a load beam 6. Also, the magnetic head 3 is incorporated with the optical head 2 located below the lower surface of disk 1 through a connecting member 4 so that a linear motor 5 may drive to move them in the radial direction of disk 1. The disk 1 is rotated by a spindle motor 7.
The optical head 2, the magnetic head 3 and the linear motor 5 will discussed further detail in the following. FIG. 2 illustrates the optical head 2 and the linear motor 5 as the top plan view thereof. Also, FIG. 3 shows cross sections of the optical head 2, the magnetic head 3, the disk 1 and the linear motor 5 (along the radial direction of the disk).
The optical head 2 has an unrepresented laser source, which emits a light beam. The objective lens 103 focuses the light beam to form a fine spot on a recording layer la in the disk 1. Focusing coils 105 and tracking coils 104 are mounted on a lens holder 107 for holding the objective lens 103 in a supported state by plate springs 108. When a current is supplied to the tracking coils 104 and the focusing coils 105, a magnetic force acts between the coils and actuator magnets 109 or actuator yokes 106 fixed on a base 100. The magnetic force moves the objective lens to scan the beam thereby so that the fine spot may accurately trace a recording track formed on the recording layer la in disk 1, performing a so-called tracking operation and focusing operation. The thus arranged actuator is mounted on a carriage 101.
The linear motor 5 is constructed of linear motor yokes 200 to each of which a linear motor magnet 201 is attached, guides 203, and linear motor coils 202 attached to the carriage 101 for the actuator. When a current is supplied to the linear motor coils 202, a magnetic force acts between the coils and the linear motor magnets 201 or the linear motor yokes 200. The magnetic force moves the carriage 101 in the radial direction of disk 1 to perform the seek operation. The carriage 101 is arranged to move on the guide rails 203 through rollers 102.
The magnetic head 3 is composed of a U-shaped core 3a made of a magnetic material, a coil 3b wound around a main pole of the core 3a, and a slider 3c made of a hard material such as ceramics. In this case, the main pole and opposed pole of core 3a are arranged to face the disk.
In recording an information signal, a current modulated according to the information signal is supplied to the coil 3b in magnetic head 3, whereby a bias field modulated according to the information signal is applied from the main pole to the recording layer 1a in disk 1 in the direction normal thereto. As described above, the optical head 2 focuses the light beam to form a fine spot irradiating a region to which the bias field is applied on the recording layer la. The temperature increases in the portion irradiated by the light beam in the recording layer 1a to suddenly decrease the coercive force therein. Then magnetization is applied to perform recording corresponding to the direction of the applied bias field. As the disk is further rotated, the thus magnetized portion leaves the portion irradiated by the light beam, whereby the temperature again decreases. With the temperature decrease, the coercive force suddenly increases to maintain the recorded magnetization. In this manner, information is recorded in a pattern of magnetization corresponding to the information signal.
As described above, electromagnetic drive means is used in general as the actuator or the linear motor for an optical head in a magneto-optical recording apparatus. In this arrangement, magnetism could leak from magnets in the drive means, and the leaking magnetism sometimes acts near the magnetic head in recording the information signal. It was pointed out heretofore as a problem that such a leaking field had components in the direction perpendicular to the disk which was added as offset to the modulated field generated by the magnetic head and that poor signal recording could result if the recording of the information signal was carried out in the state that the field with offset was applied to the disk. A solution to this problem has already been proposed (as in Japanese Laid-open Patent Application No. 3-259444).
The inventors investigated such a problem in detail and found that influence was greater on the disk from a magnetic field caused by magnetization induced inside the core in the leaking field rather than the direct effect of the leaking field itself on the disk, for some shapes of the core.
In addition, according to the investigation by the inventor, when a core asymmetric in shape with respect to the main pole is employed, such as the substantially U-shaped core as described above, the poor signal recording is caused not only by the perpendicular components of the leaking field acting near the magnetic head to the disk but also by the magnetization induced inside the core by horizontal components.
This will be described referring to FIG. 4 and FIG. 5. FIG. 4 shows a case in which a pair of actuator magnets 109 and a pair of linear motor magnets 201 are arranged with opposed faces being opposite magnetic poles (i.e., such that the N-pole of right magnet 109b or 201b is opposed to the S-pole of left magnet 109a or 201a), in which broken lines represent schematic lines of magnetic force of leaking fields from the magnets.
FIG. 5 is an enlarged drawing to show the state of the leaking fields near the core 3a in the magnetic head shown in FIG. 4. When the arrangement of magnets as shown is employed, the direction of leaking fields as shown by broken lines is parallel to the disk 1 in the vicinity of the core 3a but there are few perpendicular components to the disk 1. In such leaking fields, magnetization is induced inside the core 3a while being distributed on the sides of main pole 3 mp and opposed pole 3 sp as shown by signs + and - in FIG. 5. A magnetic field caused by this magnetization has lines of magnetic force as shown by solid lines in FIG. 5, which leave the ends of main pole 3 mp of core 3a and reach the ends of the opposed pole 3 sp. This magnetic field acts in the perpendicular direction to the portion irradiated by the light beam in the recording layer 1a in disk 1 immediately below the main pole 3 mp. Consequently, even if the leaking fields from the actuator magnets and from the linear motor magnets have only the parallel components but no perpendicular components relative to the disk, the magnetic field caused by the magnetization of the core acts also in the perpendicular direction relative to the disk, which is added as an offset in recording the information signal and therefore could be a cause of the poor signal recording.
The above phenomenon appears outstanding if the shape of the core is asymmetric with respect to the main pole. Actually, the shape of the core is not symmetric with respect to the main pole in most cases because of issues in production. For example, the core is usually formed approximately in a U shape, as shown, including a ring head. Therefore, such a problem is very serious.
Here, if the main pole and the opposed pole in the approximately U-shaped core are aligned in the tangential direction of the disk as in the above example, the above problem is rarely caused by a radial component on the disk (which is a component perpendicular to the plane of FIG. 4 or FIG. 5) among the parallel components to the disk in the leaking field. The above problem is mainly caused by a tangential component to the disk (which is a parallel component to the plane of FIG. 4 or FIG. 5).